Nd—Fe—B magnets have been recently developed as the leading RE permanent magnets with the highest room temperature magnetic properties beneficial for the wide use. The experimental value of the energy product of sintered Nd—Fe—B reached 59.5 MGOe about 93% of the theoretic value in 2006, which was attained through the conventional single-alloy powder metallurgy method. Total weight of the 2006 production of Nd—Fe—B sintered magnets probably reached 50000 metric tones.
But the Nd—Fe—B rare earth permanent magnets are susceptible to oxidation. For conventional sintered Nd—Fe—B magnet, its poor corrosion resistance in various environments is thought to be due to its complex microstructure. In detail, apart from the coarse and uneven Nd2Fe14B main phase grains, the chemically active netlike Nd-rich grain boundary phase plays an important role in the corrosion process, during which it serves as an effective pathways of intergranular corrosion propagation. As shown in the Table.1, the high chemical activity and the network structure of the Nd-rich phase are mainly responsible for the poor corrosion resistance of these alloys.
TABLE 1The composition and electrostatic potential ofthe main phases of Nd—Fe—B magnetElectrostaticpotentialPhaseFeatureV(Ag/AgCl)matrix phasePolygonal, different sizes−0.515B-rich phaseParticle precipitation≈−0.46Nd-rich phaseDistribute along the grain≈−0.65boundaries
The high content of neodymium as one of the most reactive elements may contribute to the high surface disintegration. Such an intergranular mode of corrosion results in irreversible loss in coercivity, contamination, and even total disintegration. The schematic illustration of the electrochemical corrosion of the sintered Nd—Fe—B magnet is shown in FIG. 1. Numerous researches have been carried out to improve their corrosion resistance, either by adding alloying elements to provide better inherent corrosion resistance, or by applying protective coatings on finished magnets.
Many investigations have studied the effect of alloying additions on the magnetic properties and corrosion behavior of NdFeB magnets. The additions can be divided into two groups: (1) Partial substitution of Nd by rare-earth (RE) metals, e.g. Dy, Pr and Tb. (2) Partial substitution of Fe by transition metals and main group elements, e.g. Al, Co, Cr, Cu, Mo, Nb, Ga, Ti, Zr and W. Dy, Pr and Tb additions exert no beneficial effect on the corrosion behavior, whereas, Al, Co, Cu and Ga additions are found to improve the corrosion resistance of NdFeB magnets in many corrosive environments. The improvement in the corrosion resistance is attributed to the change in the microstructure and the segregation of these kinds of additions into intergranular phase regions. It is believed that this microstructure restricted pathways for corrosion propagation through the magnet and effectively suppressed intergranular corrosion process along the intergranular phase. Nevertheless, the addition of alloying elements usually produces an improvement of corrosion resistance at a cost of impairing other properties. The reason is that one or several of the intrinsic magnetic properties of the matrix phase are impaired as these elements are dissolved in the matrix phase.
Furthermore, surface treatment technologies such as nickel electroplating, zinc electroplating, hot dip zinc, nickel electroless plating, electrophoresis, chromate-passivated aluminum coating, organic coating are currently used in corrosion-protecting for NdFeB magnet. Each technology mentioned above has its own shortcomings in applying to NdFeB, such as environmentally unfriendly and higher cost. Therefore, the best way for protecting the magnets from the attacks by climatic and corrosive environments is to improve the intrinsic corrosion resistance.
H. R. Madaah Hosseini et al. produced anisotropic (Nd, MM)2(Fe, Co, Ni)14B-type sintered magnets (MM: denotes a Misch-metal) by the binary powder blending technique (BPBT). The composition of the master alloy was close to the stoichiometric Nd2Fe14B compound, while the sintering aid (SA) had a composition of MM38.1Co46.4Ni15.4. The composition of the MM was 50 wt % Ce-27 wt % La-16 wt % Nd-7 wt % Pr. The magnets were made by blending different ratios of the master alloy and the sintering aid. The corrosion behavior of these magnets was compared with that of the Nd17Fe75B8 base alloy by potentiodynamic polarization measurements in H2SO4 solution. Compared with the conventional sintered magnet, the magnets possess higher corrosion resistance, which led to less reduction of magnetic properties of the BPBT magnet than that of the conventional sintered magnet. But the amount of Nd-rich grain boundary phases and the electrochemical potential difference between ferromagnetic and intergranular phases reduced little attributed to the high RE-content.
Based on the argumentation, it is necessary to find an alloy (or a method) for improving not only the intrinsic corrosion resistance for coating-free application but also the magnetic performances (Br and (BH)max).